The present invention relates to an internal combustion engine fuel injector.
As is known, an injector comprises an injector body, which defines a nozzle for injecting fuel into the engine, and houses a control rod movable along an axis to activate a pin closing the nozzle. The injector body also houses an electric control servo valve comprising a control chamber bounded axially at one side by the control rod and at the other side by an end wall having an outlet hole, which is opened/closed by a shutter to communicate with a discharge conduit and so vary the pressure in the control chamber. More specifically, the cross section of the outlet hole is calibrated to accurately set fuel flow from the control chamber to the discharge conduit, and the shutter is movable axially under the control of an electro-actuator and the axial thrust of a spring, which is preloaded to keep the outlet hole closed when the electro-actuator is idle.
A need is felt for injectors in which the shutter opening/closing the outlet hole of the control chamber is subjected to substantially zero pressure when the shutter is in the closed position, so as to reduce the preload of the spring, the force required of the electro-actuator, and therefore size, as compared with solutions in which the shutter closes the outlet hole axially. More specifically, in injectors in which the shutter is “balanced” in terms of axial pressure, even a small amount of lift of the shutter produces a large fuel flow section to the discharge conduit, thus improving dynamic performance of the injector, i.e. by eliminating so-called “bounce” of the shutter at the end of the opening and closing strokes.
At the same time, a need is also felt for an injector which, in addition to a “balanced” shutter, provides for minimizing variations in opening/closing performance of the injection nozzle with respect to design conditions.
It is an object of the present invention to provide an internal combustion engine fuel injector designed to meet the above demands in a straightforward, low-cost manner, and which, in particular, is of relatively straightforward, compact construction.
According to the present invention, there is provided a fuel injector for an internal combustion engine; the injector terminating with a nozzle for injecting fuel into a relative cylinder of the engine, and comprising:                a hollow injector body extending in an axial direction;        a control rod movable axially with respect to said injector body to open/close said nozzle;        a control servo valve housed in said injector body and comprising:        a) an electro-actuator;        b) a control chamber, which is bounded axially at one side by said control rod, communicates with a fuel inlet, and has a discharge passage comprising a calibrated portion; and        c) a shutter movable axially, under the control of said electro-actuator, between a closed position, in which it closes said discharge passage, and an open position, in which it opens said discharge passage to vary the pressure in said control chamber and so produce axial movement of said control rod;        
characterized in that:                said control servo valve also comprises an axial rod fixed with respect to said injector body and comprising an outer lateral surface through which said discharge passage comes out;        said shutter is fitted to said outer lateral surface to slide axially in substantially fluidtight manner, and, in said closed position, closes said discharge passage so as to be subjected to substantially zero resultant axial pressure by the fuel; and        said calibrated portion is so formed as to produce swirl and/or cavitation in the fuel outflow near to the closing area between said shutter and said axial rod.        